Why are some people lactose intolerant?

Answer 1

Lactose intolerance is the basic condition of most of the human race. Only on Europe and some parts of Africa were food supplies so limited that this with the mutation that allowed them to consume milk proved to be beneficial and those with it didn't starve to death (or at least had better breeding success)So the answer is that those that were lactose intolerant come from a population that never had codevelop the ability or those who by breeding picked up the "normal" lactose intolerant gene structure from their varied ancestors.

Answer 2

Their body lacks- or lost- the ability to digest lactose (milk sugar).